1. Field of the Invention
The present invention relates to a tape or sheet form pressure-sensitive adhesive material having increased adhesive force, namely adhesive strength under shear and peel strength. More particularly, the present invention relates to a pressure-sensitive adhesive tape or sheet comprising a substrate with flexibility and high tensile strength which substrate is prepared by curing a specific resin composition and a pressure-sensitive adhesive layer laminated on said substrate.
2. Description of the Related Art
There are some techniques to provide a pressure-sensitive adhesive tape or sheet with comparatively high adhesive power.
For example, it was proposed to increase the thickness of the pressure-sensitive adhesive layer on the substrate to produce the pressure-sensitive adhesive material with comparatively high adhesive power. However, the adhesive force of such pressure-sensitive adhesive material has its own limit since the thickness of the pressure-sensitive adhesive layer has its upper limit insofar as the adhesive layer is formed from a solution or emulsion type adhesive mass.
The use of a thick foam material as the substrate improves the adhesive force of the pressure-sensitive adhesive material. In case of most commonly used polyurethane or polyethylene foam, as the adhesive strength under shear is increased, the peel strength is decreased, while as the peel strength is increased, the adhesive strength under shear is decreased. Accordingly, the pressure-sensitive adhesive material comprising the foam substrate has its limit in the adhesive strength under shear, and it is very difficult to impart both comparatively high peel strength and comparatively high adhesive strength under shear to the pressure-sensitive adhesive material. Thus, none of such the foam type pressure-sensitive adhesive materials has sufficient performances.
To solve the above problems, it is proposed to produce a pressure-sensitive adhesive film containing a certain amount of bubbles of specific gas, whereby a pressure-sensitive adhesive material having both flexibility and comparatively high adhesive force is provided (cf. Japanese Patent Kokai Publication No. 125776/1983). However, this pressure-sensitive adhesive material has various drawbacks in its structure and manufacture, since it is a very special type adhesive which contains the certain amount of the bubble of specific gas and it should be produced by a method far different from the conventional methods, which method comprises whipping a composition having a pressure-sensitive adhesive property through polymerization, placing the whipped composition on the substrate and then in situ polymerizing the composition before the bubbles disappear.
It is also suggested to use minute glass balloons in place of the gas bubbles in the adhesive film (cf. Japanese Patent Kokai Publication Nos. 141346/1978 and 34976/1987). However, since the glass balloons tends to be easily broken during mixing, they should be carefully handled and processed.